Planetary gear train of automatic transmission for vehicle

ABSTRACT

A planetary gear train of an automatic transmission for a vehicle may include an input shaft receiving torque of an engine, an output shaft outputting torque, a first planetary gear set, a second planetary gear set, a third planetary gear set, a fourth planetary gear set, a first shaft, a second shaft, a third shaft, a fourth shaft selectively connectable to the second shaft, and directly connected to the input shaft, a fifth shaft, a sixth shaft, a seventh shaft selectively connectable to the second shaft, and an eighth shaft selectively connectable to the third shaft, and directly connected to the output shaft.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to Korean Patent Application No.10-2016-0080270, filed Jun. 27, 2016, the entire contents of which isincorporated herein for all purposes by this reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to an automatic transmission for avehicle. More particularly, the present invention relates to a planetarygear train of an automatic transmission for a vehicle that improvespower delivery performance and fuel economy by achieving ten forwardspeed stages with a minimum number of constituent elements being usedand improves silent driving of the vehicle by using an operation pointpositioned at a low engine speed.

Description of Related Art

Generally, an automatic transmission achieving more speed stages hasbeen developed to enhance fuel economy and optimize drivability. Recentincreases in oil prices are triggering stiff competition in enhancingfuel consumption of a vehicle.

Therefore, much research for reducing weight and enhancing fuel economythrough downsizing of an engine and for securing drivability and fueleconomy through multiple speed stages of automatic transmissions hasbeen developed.

However, in the automatic transmission, as the number of speed stagesincrease, the number of internal components (particularly, planetarygear sets) increase, and as a result, a length of the transmissionincreases. This may deteriorate installability and/or power flowefficiency and may increase production cost, and weight.

Accordingly, development of a planetary gear train which may achievemaximum efficiency with a small number of components is important inorder to increase a fuel economy enhancement effect through the multiplespeed-stages.

In this aspect, in recent years, 8-speed automatic transmissions tend tobe implemented and the research and development of a planetary geartrain capable of implementing more speed stages has also been activelyconducted.

However, a conventional 8-speed automatic transmission typicallyincludes three to four planetary gear sets and five to six controlelements (frictional elements). In this case, since the length of theautomatic transmission increases, mountability may be deteriorated.

In recent planetary gear sets, one planetary gear set is disposed aboveanother planetary gear set, but structures of automatic transmissions towhich parallel planetary gear sets is applied are very limited.

In other planetary gear sets, dog clutches instead of control elementsof wet-type are used. However, shift feel may be deteriorated.

The information disclosed in this Background of the Invention section isonly for enhancement of understanding of the general background of theinvention and should not be taken as an acknowledgement or any form ofsuggestion that this information forms the prior art already known to aperson skilled in the art.

BRIEF SUMMARY

Various aspects of the present invention are directed to providing aplanetary gear train of an automatic transmission for a vehicle havingadvantages of improving power delivery performance and fuel economy byachieving at least ten forward speed stages and one reverse speed stage.

Additionally, various aspects of the present invention are directed toproviding a planetary gear train of an automatic transmission for avehicle having further advantages of improving silent driving of thevehicle by using operation point positioned at a low rotation speedregion of an engine.

According to various aspects of the present invention, a planetary geartrain of an automatic transmission for a vehicle may include an inputshaft receiving torque of an engine, an output shaft outputting torque,a first planetary gear set including first, second, and third rotationelements, a second planetary gear set including fourth, fifth, and sixthrotation elements, a third planetary gear set including seventh, eighth,and ninth rotation elements, a fourth planetary gear set includingtenth, eleventh, and twelfth rotation elements, a first shaft connectedto the first rotation element, a second shaft connecting the secondrotation element to the fifth rotation element, a third shaft connectingthe third rotation element to the eighth rotation element, a fourthshaft connecting the fourth rotation element to the seventh rotationelement, selectively connectable to the second shaft, and directlyconnected to the input shaft, a fifth shaft connected to the sixthrotation element, a sixth shaft connecting the ninth rotation element tothe eleventh rotation element, a seventh shaft connected to the tenthrotation element and selectively connectable to the second shaft, and aneighth shaft connected to the twelfth rotation element, selectivelyconnectable to the third shaft, and directly connected to the outputshaft.

The first shaft, the fifth shaft and the seventh shaft may each beselectively connectable to a transmission housing.

The first, second, and third rotation elements may be a first sun gear,a first planet carrier, and a first ring gear, the fourth, fifth, andsixth rotation elements may be a second sun gear, a second planetcarrier, and a second ring gear, the seventh, eighth, and ninth rotationelements may be a third sun gear, a third planet carrier, and a thirdring gear, and the tenth, eleventh, and twelfth rotation elements may bea fourth sun gear, a fourth planet carrier, and a fourth ring gear.

The planetary gear train may further include a first clutch selectivelyconnecting the second shaft to the fourth shaft, a second clutchselectively connecting the third shaft to the eighth shaft, a thirdclutch selectively connecting the second shaft to the seventh shaft, afirst brake selectively connecting the first shaft to the transmissionhousing, a second brake selectively connecting the fifth shaft to thetransmission housing, and a third brake selectively connecting theseventh shaft to the transmission housing.

According to various aspects of the present invention, a planetary geartrain of an automatic transmission for a vehicle may include an inputshaft receiving torque of an engine, an output shaft outputting torque,a first planetary gear set including first, second, and third rotationelements, a second planetary gear set including fourth, fifth, and sixthrotation elements, a third planetary gear set including seventh, eighth,and ninth rotation elements, and a fourth planetary gear set includingtenth, eleventh, and twelfth rotation elements, in which the input shaftmay be directly connected to the fourth rotation element, the outputshaft may be directly connected to the twelfth rotation element, thesecond rotation element may be directly connected to the fifth rotationelement, the third rotation element may be directly connected to theeighth rotation element, the seventh rotation element may be directlyconnected to the fourth rotation element and may be selectivelyconnectable to the fifth rotation element, the ninth rotation elementmay be directly connected to the eleventh rotation element, the tenthrotation element may be selectively connectable to the second rotationelement, and the twelfth rotation element may be selectively connectableto the eighth rotation element.

The first rotation element, the sixth rotation element and the tenthrotation element may each be selectively connectable to a transmissionhousing.

The planetary gear train may further include a first clutch selectivelyconnecting the fifth rotation element to the seventh rotation element, asecond clutch selectively connecting the eighth rotation element to thetwelfth rotation element, a third clutch selectively connecting thesecond rotation element to the tenth rotation element, a first brakeselectively connecting the first rotation element to the transmissionhousing, a second brake selectively connecting the sixth rotationelement to the transmission housing, and a third brake selectivelyconnecting the tenth rotation element to the transmission housing.

Speed stages implemented by selectively operating the first, second, andthird clutches and the first, second, and third brakes may include afirst forward speed stage, implemented by operating the third clutch,and the first and second brakes, a second forward speed stage,implemented by operating the first, second and third brakes, a thirdforward speed stage, implemented by operating the second clutch, and thefirst and second brakes, a fourth forward speed stage, implemented byoperating the second clutch, and the second and third brakes, a fifthforward speed stage, implemented by operating the second and thirdclutches, and the second brake, a sixth forward speed stage, implementedby operating the second and third clutches and the first brake, aseventh forward speed stage, implemented by operating the first, secondand third clutches, an eighth forward speed stage, implemented byoperating the first and second clutches, and the first brake, a ninthforward speed stage, implemented by operating the first and thirdclutches, and the first brake, a tenth forward speed stage, implementedby operating the first clutch, and the first and third brakes, and areverse-speed stage, implemented by operating the third clutch, and thefirst and third brakes.

The planetary gear train according to various embodiments of the presentinvention achieves ten forward speed stages and one reverse speed stageby combining four planetary gear sets with six control elements.

In addition, the planetary gear train according to various embodimentsof the present invention achieves speed stages suitable to a rotationspeed of the engine due to multiple-speed stages of the automatictransmission. Particularly, silent driving of the vehicle is improved byusing an operation point positioned at a low rotation speed region of anengine.

In addition, the planetary gear train according to various embodimentsof the present invention maximizes driving efficiency of the engine andimproves power delivery performance and fuel consumption.

It is understood that the term “vehicle” or “vehicular” or other similarterms as used herein is inclusive of motor vehicles in general such aspassenger automobiles including sports utility vehicles (SUV), buses,trucks, various commercial vehicles, watercraft including a variety ofboats and ships, aircraft, and the like, and includes hybrid vehicles,electric vehicles, plug-in hybrid electric vehicles, hydrogen-poweredvehicles and other alternative fuel vehicles (e.g., fuel derived fromresources other than petroleum). As referred to herein, a hybrid vehicleis a vehicle that has two or more sources of power, for example, bothgasoline-powered and electric-powered vehicles.

The methods and apparatuses of the present invention have other featuresand advantages which will be apparent from or are set forth in moredetail in the accompanying drawings, which are incorporated herein, andthe following Detailed Description, which together serve to explaincertain principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a planetary gear train according tovarious embodiments of the present invention.

FIG. 2 is an operation chart of control elements at each speed stage inthe planetary gear train according to various embodiments of the presentinvention.

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variousfeatures illustrative of the basic principles of the invention. Thespecific design features of the present invention as disclosed herein,including, for example, specific dimensions, orientations, locations,and shapes will be determined in part by the particular intendedapplication and use environment.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments of thepresent invention(s), examples of which are illustrated in theaccompanying drawings and described below. While the invention(s) willbe described in conjunction with exemplary embodiments, it will beunderstood that the present description is not intended to limit theinvention(s) to those exemplary embodiments. On the contrary, theinvention(s) is/are intended to cover not only the exemplaryembodiments, but also various alternatives, modifications, equivalentsand other embodiments, which may be included within the spirit and scopeof the invention as defined by the appended claims.

FIG. 1 is a schematic diagram of a planetary gear train according tovarious embodiments of the present invention.

Referring to FIG. 1, a planetary gear train according to variousembodiments of the present invention includes first, second, third, andfourth planetary gear sets PG1, PG2, PG3, and PG4 disposed on the sameaxis, an input shaft IS, an output shaft OS, eight rotation shafts TM1to TM8 connected to at least one of rotation elements of the first,second, third, and fourth planetary gear sets PG1, PG2, PG3, and PG4,three clutches C1 to C3 and three brakes B1 to B3 that are controlelements, and a transmission housing H.

Torque input from the input shaft IS is changed by cooperation of thefirst, second, third, and fourth planetary gear sets PG1, PG2, PG3, andPG4 and the changed torque is output through the output shaft OS.

The planetary gear sets are disposed in the order of first, second,third and fourth planetary gear sets PG1, PG2, PG3 and PG4 from anengine side.

The input shaft IS is an input member and the torque from a crankshaftof an engine, after being torque-converted through a torque converter,is input into the input shaft IS.

The output shaft OS is an output member, is disposed in parallel withthe input shaft IS, and transmits driving torque to a driving wheelthrough a differential apparatus.

The first planetary gear set PG1 is a single pinion planetary gear setand includes a first sun gear S1, a first planet carrier PC1 rotatablysupporting a first pinion P1 that is externally meshed with the firstsun gear S1, and a first ring gear R1 that is internally meshed with thefirst pinion P1 respectively as first, second, and third rotationelements N1, N2, and N3.

The second planetary gear set PG2 is a single pinion planetary gear setand includes a second sun gear S2, a second planet carrier PC2 rotatablysupporting a second pinion P2 that is externally meshed with the secondsun gear S2, and a second ring gear R2 that is internally meshed withthe second pinion P2 respectively as fourth, fifth, and sixth rotationelements N4, N5, and N6.

The third planetary gear set PG3 is a single pinion planetary gear setand includes a third sun gear S3, a third planet carrier PC3 rotatablysupporting a third pinion P3 that is externally meshed with the thirdsun gear S3, and a third ring gear R3 that is internally meshed with thethird pinion P3 respectively as seventh, eighth, and ninth rotationelements N7, N8, and N9.

The fourth planetary gear set PG4 is a single pinion planetary gear setand includes a fourth sun gear S4, a fourth planet carrier PC4 rotatablysupporting a fourth pinion P4 that is externally meshed with the fourthsun gear S4, and a fourth ring gear R4 that is internally meshed withthe fourth pinion P4 respectively as tenth, eleventh, and twelfthrotation elements N10, N11, and N12.

The second rotation element N2 is directly connected to the fifthrotation element N5, the third rotation element N3 is directly connectedto the eighth rotation element N8, the fourth rotation element N4 isdirectly connected to the seventh rotation element N7, and the ninthrotation element N9 is directly connected to the eleventh rotationelement N11 such that the first, second, third, and fourth planetarygear sets PG1, PG2, PG3, and PG4 include eight rotation shaft TM1 toTM8.

The eight rotation shafts TM1 to TM8 will be described in detail.

The eight shafts TM1 to TM8 directly connect a plurality of rotationelements among the rotation elements of the planetary gear sets PG1,PG2, PG3, and PG4, are rotation members that are each directly connectedto at least one rotation element of the planetary gear sets PG1, PG2,PG3, and PG4 and rotate with the at least one rotation element totransmit torque, or are fixed members that directly connect at least onerotation element of the planetary gear sets PG1, PG2, PG3, and PG4 tothe transmission housing H to fix the at least one rotation element.

The first shaft TM1 is connected to the first rotation element N1 (firstsun gear S1) and is selectively connectable to the transmission housingH so as to be operated as a selective fixed element.

The second shaft TM2 connects the second rotation element N2 (firstplanet carrier PC1) to the fifth rotation element N5 (second planetcarrier PC2).

The third shaft TM3 connects the third rotation element N3 (first ringgear R1) to the eighth rotation element N8 (third planet carrier PC3).

The fourth shaft TM4 connects the fourth rotation element N4 (second sungear S2) to the seventh rotation element N7 (third sun gear S3), isselectively connectable to the second shaft TM2, and is directlyconnected to the input shaft IS so as to be continuously operated as aninput element.

The fifth shaft TM5 is connected to the sixth rotation element N6(second ring gear R2) and is selectively connectable to the transmissionhousing H so as to be operated as a selective fixed element.

The sixth shaft TM6 connects the ninth rotation element N9 (third ringgear R3) to the eleventh rotation element N11 (fourth planet carrierPC4).

The seventh shaft TM7 is connected to the tenth rotation element N10(fourth sun gear S4), is selectively connectable to the second shaftTM2, and is selectively connectable to the transmission housing H so asto be operated as a selective fixed element.

The eighth shaft TM8 is connected to the twelfth rotation element N12(fourth ring gear R4), is selectively connectable to the third shaftTM3, and is directly connected to the output shaft OS so as to becontinuously operated as an output element.

In addition, three clutches C1, C2, and C3 are disposed at positions atwhich any two shafts among the eight shafts TM1 to TM8 including theinput shaft IS and the output shaft OS are selectively connectable toeach other.

In addition, three brakes B1, B2, and B3 are disposed at positions atwhich at least one shaft among the eight shafts TM1 to TM8 isselectively connectable to the transmission housing H.

Arrangements of the three clutches C1 to C3 and the three brakes B1 toB3 are described in detail.

The first clutch C1 is disposed between the second shaft TM2 and thefourth shaft TM4 and selectively connects the second shaft TM2 to thefourth shaft TM4.

The second clutch C2 is disposed between the third shaft TM3 and theeighth shaft TM8 and selectively connects the third shaft TM3 to theeighth shaft TM8.

The third clutch C3 is disposed between the second shaft TM2 and theseventh shaft TM7 and selectively connects the second shaft TM2 to theseventh shaft TM7.

The first brake B1 is disposed between the first shaft TM1 and thetransmission housing H and selectively connects the first shaft TM1 tothe transmission housing H.

The second brake B2 is disposed between the fifth shaft TM5 and thetransmission housing H and selectively connects the fifth shaft TM5 tothe transmission housing H.

The third brake B3 is disposed between the seventh shaft TM7 and thetransmission housing H and selectively connects the seventh shaft TM7 tothe transmission housing H.

The control elements including the first, second, and third clutches C1,C2, and C3 and the first, second, and third brakes B1, B2, and B3 may bemulti-plates friction elements of wet type that are operated byhydraulic pressure.

FIG. 2 is an operation chart of control elements at each speed stage inthe planetary gear train according to various embodiments of the presentinvention.

As shown in FIG. 2, three control elements among the first, second, andthird clutches C1, C2, and C3 and the first, second, and third brakesB1, B2, and B3 that are control elements are operated at each speedstage in the planetary gear train according to various embodiments ofthe present invention.

The third clutch C3 and the first and second brakes B1 and B2 areoperated at a first forward speed stage D1.

In a state that the second shaft TM2 is connected to the seventh shaftTM7 by operation of the third clutch C3, torque of the input shaft IS isinput to the fourth shaft TM4.

In addition, the first shaft TM1 and the fifth shaft TM5 are operated asthe fixed elements by operation of the first and second brakes B1 andB2. Therefore, the torque of the input shaft IS is shifted into thefirst forward speed stage, and the first forward speed stage is outputthrough the output shaft OS connected to the eighth shaft TM8.

The first, second, and third brakes B1, B2, and B3 are operated at asecond forward speed stage D2.

The torque of the input shaft IS is input to the fourth shaft TM4.

In addition, the first shaft TM1, the fifth shaft TM5, and the seventhshaft TM7 are operated as the fixed elements by operation of the first,second, and third brakes B1, B2, and B3. Therefore, the torque of theinput shaft IS is shifted into the second forward speed stage, and thesecond forward speed stage is output through the output shaft OSconnected to the eighth shaft TM8.

The second clutch C2 and the first and second brakes B1 and B2 areoperated at a third forward speed stage D3.

In a state that the third shaft TM3 is connected to the eighth shaft TM8by operation of the second clutch C2, the torque of the input shaft ISis input to the fourth shaft TM4.

In addition, the first shaft TM1 and the fifth shaft TM5 are operated asthe fixed elements by operation of the first and second brakes B1 andB2. Therefore, the torque of the input shaft IS is shifted into thethird forward speed stage, and the third forward speed stage is outputthrough the output shaft OS connected to the eighth shaft TM8.

The second clutch C2 and the second and third brakes B2 and B3 areoperated at a fourth forward speed stage D4.

In a state that the third shaft TM3 is connected to the eighth shaft TM8by operation of the second clutch C2, the torque of the input shaft ISis input to the fourth shaft TM4.

In addition, the fifth shaft TM5 and the seventh shaft TM7 are operatedas the fixed elements by operation of the second and third brakes B2 andB3. Therefore, the torque of the input shaft IS is shifted into thefourth forward speed stage, and the fourth forward speed stage is outputthrough the output shaft OS connected to the eighth shaft TM8.

The second and third clutches C2 and C3 and the second brake B2 areoperated at a fifth forward speed stage D5.

In a state that the third shaft TM3 is connected to the eighth shaft TM8by operation of the second clutch C2 and the second shaft TM2 isconnected to the seventh shaft TM7 by operation of the third clutch C3,the torque of the input shaft IS is input to the fourth shaft TM4.

In addition, the fifth shaft TM5 is operated as the fixed element byoperation of the second brake B2. Therefore, the torque of the inputshaft IS is shifted into the fifth forward speed stage, and the fifthforward speed stage is output through the output shaft OS connected tothe eighth shaft TM8.

The second and third clutches C2 and C3 and the first brake B1 areoperated at a sixth forward speed stage D6.

In addition, the third shaft TM3 is connected to the eighth shaft TM8 byoperation of the second clutch C2 and the second shaft TM2 is connectedto the seventh shaft TM7 by operation of the third clutch C3, the torqueof the input shaft IS is input to the fourth shaft TM4.

In addition, the first shaft TM1 is operated as the fixed element byoperation of the first brake B1. Therefore, the torque of the inputshaft IS is shifted into the sixth forward speed stage, and the sixthforward speed stage is output through the output shaft OS connected tothe eighth shaft TM8.

The first, second, and third clutches C1, C2, and C3 are operated at aseventh forward speed stage D7.

In a state that the second shaft TM2 is connected to the fourth shaftTM4 by operation of the first clutch C1, the third shaft TM3 isconnected to the eighth shaft TM8 by operation of the second clutch C2,and the second shaft TM2 is connected to the seventh shaft TM7 byoperation of the third clutch C3, the torque of the input shaft IS isinput to the fourth shaft TM4 and the second shaft TM2.

In this case, the first, second, third, and fourth planetary gear setsPG1, PG2, PG3, and PG4 become lock-up states and the seventh forwardspeed stage is output through the output shaft OS connected to theeighth shaft TM8. At the seventh forward speed stage, the same rotationspeed as the input shaft IS is output.

The first and second clutches C1 and C2 and the first brake B1 areoperated at an eighth forward speed stage D8.

In a state that the second shaft TM2 is connected to the fourth shaftTM4 by operation of the first clutch C1 and the third shaft TM3 isconnected to the eighth shaft TM8 by operation of the second clutch C2,the torque of the input shaft IS is input to the fourth shaft TM1 andthe second shaft TM2.

In addition, the first shaft TM1 is operated as the fixed element byoperation of the first brake B1. Therefore, the torque of the inputshaft IS is shifted into the eighth forward speed stage, and the eighthforward speed stage is output through the output shaft OS connected tothe eighth shaft TM8.

The first and third clutches C1 and C3 and the first brake B1 areoperated at a ninth forward speed stage D9.

In a state that the second shaft TM2 is connected to the fourth shaftTM4 and the seventh shaft TM7 by operation of the first and thirdclutches C1 and C3, the torque of the input shaft IS is input to thefourth shaft TM4 and the second shaft TM2.

In addition, the first shaft TM1 is operated as the fixed element byoperation of the first brake B1. Therefore, the torque of the inputshaft IS is shifted into the ninth forward speed stage, and the ninthforward speed stage is output through the output shaft OS connected tothe eighth shaft TM8.

The first clutch C1 and the first and third brakes B1 and B3 areoperated at a tenth forward speed stage D10.

In a state that the second shaft TM2 is connected to the fourth shaftTM4 by operation of the first clutch C1, the torque of the input shaftIS is input to the fourth shaft TM4 and the second shaft TM2.

In addition, the first shaft TM1 and the seventh shaft TM7 are operatedas the fixed elements by operation of the first and third brakes B1 andB3. Therefore, the torque of the input shaft IS is shifted into thetenth forward speed stage, and the tenth forward speed stage is outputthrough the output shaft OS connected to the eighth shaft TM8.

The third clutch C3 and the first and third brakes B1 and B3 areoperated at a reverse speed stage REV.

In a state that the second shaft TM2 is connected to the seventh shaftTM7 by operation of the third clutch C3, the torque of the input shaftIS is input to the fourth shaft TM4.

In addition, the first shaft TM1 and the seventh shaft TM7 are operatedas the fixed elements by operation of the first and third brakes B1 andB4. Therefore, the torque of the input shaft IS is shifted into thereverse speed stage, and the reverse speed stage is output through theoutput shaft OS connected to the eighth shaft TM8.

The planetary gear trains according to various embodiments of thepresent invention achieves at least ten forward speed stages and onereverse speed stage by combining four planetary gear sets PG1, PG2, PG3,and PG4 with the three clutches C1, C2, and C3 and the three brakes B1,B2, and B3.

In addition, the planetary gear train according to various embodimentsof the present invention achieves suitable speed stages according torotation speed of the engine. Particularly, silent driving of thevehicle is improved by using operation point positioned at a lowrotation speed region of the engine.

Furthermore, the planetary gear train according to various embodimentsof the present invention maximizes driving efficiency of the engine andimproves power delivery performance and fuel consumption.

The foregoing descriptions of specific exemplary embodiments of thepresent invention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and obviously manymodifications and variations are possible in light of the aboveteachings. The exemplary embodiments were chosen and described in orderto explain certain principles of the invention and their practicalapplication, to thereby enable others skilled in the art to make andutilize various exemplary embodiments of the present invention, as wellas various alternatives and modifications thereof. It is intended thatthe scope of the invention be defined by the Claims appended hereto andtheir equivalents.

What is claimed is:
 1. A planetary gear train of an automatictransmission for a vehicle comprising: an input shaft receiving torqueof an engine; an output shaft outputting torque; a first planetary gearset including a first rotation element, a second rotation element, and athird rotation element; a second planetary gear set including a fourthrotation element, a fifth rotation element, and a sixth rotationelement; a third planetary gear set including a seventh rotationelement, an eighth rotation element, and a ninth rotation element; afourth planetary gear set including a tenth rotation element, aneleventh rotation element, and a twelfth rotation element; a first shaftconnected to the first rotation element; a second shaft connecting thesecond rotation element to the fifth rotation element; a third shaftconnecting the third rotation element to the eighth rotation element; afourth shaft connecting the fourth rotation element to the seventhrotation element, selectively connectable to the second shaft, anddirectly connected to the input shaft; a fifth shaft connected to thesixth rotation element; a sixth shaft connecting the ninth rotationelement to the eleventh rotation element; a seventh shaft connected tothe tenth rotation element and selectively connectable to the secondshaft; and an eighth shaft connected to the twelfth rotation element,selectively connectable to the third shaft, and directly connected tothe output shaft.
 2. The planetary gear train of claim 1, wherein thefirst shaft, the fifth shaft and the seventh shaft are each selectivelyconnectable to a transmission housing.
 3. The planetary gear train ofclaim 1, wherein the first rotation element, the second rotationelement, and the third rotation element comprise a first sun gear, afirst planet carrier, and a first ring gear, the fourth rotationelement, the fifth rotation element, and the sixth rotation elementcomprise a second sun gear, a second planet carrier, and a second ringgear, the seventh rotation element, the eighth rotation element, and theninth rotation element comprise a third sun gear, a third planetcarrier, and a third ring gear, and the tenth rotation element, theeleventh rotation element, and the twelfth rotation element comprise afourth sun gear, a fourth planet carrier, and a fourth ring gear.
 4. Theplanetary gear train of claim 2, further comprising: a first clutchselectively connecting the second shaft to the fourth shaft; a secondclutch selectively connecting the third shaft to the eighth shaft; athird clutch selectively connecting the second shaft to the seventhshaft; a first brake selectively connecting the first shaft to thetransmission housing; a second brake selectively connecting the fifthshaft to the transmission housing; and a third brake selectivelyconnecting the seventh shaft to the transmission housing.
 5. A planetarygear train of an automatic transmission for a vehicle comprising: aninput shaft receiving torque of an engine; an output shaft outputtingtorque; a first planetary gear set including a first rotation element, asecond rotation element, and a third rotation element; a secondplanetary gear set including a fourth rotation element, a fifth rotationelement, and a sixth rotation element; a third planetary gear setincluding a seventh rotation element, an eighth rotation element, and aninth rotation element; and a fourth planetary gear set including atenth rotation element, an eleventh rotation element, and a twelfthrotation element, wherein the input shaft is directly connected to thefourth rotation element, the output shaft is directly connected to thetwelfth rotation element, the second rotation element is directlyconnected to the fifth rotation element, the third rotation element isdirectly connected to the eighth rotation element, the seventh rotationelement is directly connected to the fourth rotation element and isselectively connectable to the fifth rotation element, the ninthrotation element is directly connected to the eleventh rotation element,the tenth rotation element is selectively connectable to the secondrotation element, and the twelfth rotation element is selectivelyconnectable to the eighth rotation element.
 6. The planetary gear trainof claim 5, wherein the first rotation element, the sixth rotationelement and the tenth rotation element are each selectively connectableto a transmission housing.
 7. The planetary gear train of claim 5,wherein the first rotation element, the second rotation element, and thethird rotation element comprise a first sun gear, a first planetcarrier, and a first ring gear, the fourth rotation element, the fifthrotation element, and the sixth rotation element comprise a second sungear, a second planet carrier, and a second ring gear, the seventhrotation element, the eighth rotation element, and the ninth rotationelement comprise a third sun gear, a third planet carrier, and a thirdring gear, and the tenth rotation element, the eleventh rotationelement, and the twelfth rotation element comprise a fourth sun gear, afourth planet carrier, and a fourth ring gear.
 8. The planetary geartrain of claim 6, further comprising: a first clutch selectivelyconnecting the fifth rotation element to the seventh rotation element; asecond clutch selectively connecting the eighth rotation element to thetwelfth rotation element; a third clutch selectively connecting thesecond rotation element to the tenth rotation element; a first brakeselectively connecting the first rotation element to the transmissionhousing; a second brake selectively connecting the sixth rotationelement to the transmission housing; and a third brake selectivelyconnecting the tenth rotation element to the transmission housing. 9.The planetary gear train of claim 8, wherein speed stages implemented byselectively operating the first, second, and third clutches and thefirst, second, and third brakes include: a first forward speed,implemented by operating the third clutch, and the first and secondbrakes; a second forward speed, implemented by operating the first,second and third brakes; a third forward speed, implemented by operatingthe second clutch, and the first and second brakes; a fourth forwardspeed, implemented by operating the second clutch, and the second andthird brakes; a fifth forward speed, implemented by operating the secondand third clutches, and the second brake; a sixth forward speed,implemented by operating the second and third clutches and the firstbrake; a seventh forward speed, implemented by operating the first,second and third clutches; an eighth forward speed, implemented byoperating the first and second clutches, and the first brake; a ninthforward speed, implemented by operating the first and third clutches,and the first brake; a tenth forward speed, implemented by operating thefirst clutch, and the first and third brakes; and a reverse-speed stage,implemented by operating the third clutch, and the first and thirdbrakes.